Via hole depth detector

ABSTRACT

A via hole depth detector for detecting the depth of a via hole formed in a workpiece held on a chuck table, comprising: a first surface position detection means which comprises a first detection laser beam oscillation means for oscillating a first detection laser beam having a predetermined wavelength and detects the height position of an illuminated portion of the workpiece based on the reflected light of the first detection laser beam; a second surface position detection means which comprises a second detection laser beam oscillation means for oscillating a second detection laser beam having a wavelength different from the wavelength of the first detection laser beam and detects the height position of an illuminated portion of the workpiece based on the reflected light of the second detection laser beam; and a control means for obtaining the depth of the via hole formed in the workpiece based on a detection value obtained by the first surface position detection means and a detection value obtained by the second surface position detection means.

FIELD OF THE INVENTION

The present invention relates to a detector for detecting the depth of avia hole formed in a workpiece.

DESCRIPTION OF THE PRIOR ART

In the production process of a semiconductor device, a plurality ofareas are sectioned by dividing lines called “streets” arranged in alattice pattern on the front surface of a substantially disk-likesemiconductor wafer, and a device such as IC or LSI is formed in each ofthe sectioned areas. Individual semiconductor chips are manufactured bycutting this semiconductor wafer along the streets to divide it into theareas each having a device formed therein.

To reduce the size and increase the number of functions of an apparatus,a modular structure for connecting the electrodes of a plurality ofsemiconductor chips which are formed in a layer has been implemented anddisclosed by JP-A 2003-163323. This modular structure is such that viaholes reaching the electrodes are formed from the rear side at positionswhere the electrodes are formed on the front surface of a semiconductorwafer, and a conductive material such as aluminum to be connected to theelectrodes is embedded in the via holes.

The via holes formed in the above semiconductor wafer are generallyformed by a drill. However, the diameters of the via holes formed in thesemiconductor wafer are as small as 100 to 300 μm and hence, there is aproblem in that drilling the via holes is not always satisfactory interms of productivity.

To solve the above problem, the company of the applicant of the presentapplication proposed as JP-A 2006-255761 a laser beam processing machinecapable of efficiently forming via holes in a workpiece such as asemiconductor wafer. This laser beam processing machine is aprocessing-feed amount detection means for detecting the relativeprocessing-feed amount of a chuck table for holding a workpiece and alaser beam application means, a memory means for storing the X and Ycoordinate values of via holes to be formed in the workpiece and acontrol means for controlling the laser beam application means based onthe X and Y coordinate values of via holes stored in the memory meansand a detection signal from the processing-feed amount detection means,and is so constituted as to apply a laser beam when the point of the Xand Y coordinate values of a via hole to be formed in the workpiecereaches a position right below the condenser of the laser beamapplication means.

In the above method of forming via holes by applying a laser beam fromthe rear surface of the semiconductor wafer described above, theapplication of the laser beam must be stopped not to make a hole in theelectrodes formed on the front surface of the semiconductor wafer. Tothis end, the application of the laser beam must be stopped at the timewhen the via hole has reached the electrode. It is, however, extremelydifficult to detect that the via hole has reached the electrode, andthere is a case where the via hole does not have reached the electrode.Accordingly, it is necessary to check if the via hole formed in thewafer has reached the electrode or not. If the via hole does not havereached the electrode, it is desired that the depth of the via hole canbe detected so that the semiconductor wafer can be re-processedefficiently.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a detector capableof efficiently detecting the depth of a via hole formed in a workpiece.

To attain the above object, according to the present invention, there isprovided a via hole depth detector for detecting the depth of a via holeformed in a workpiece held on the holding surface of a chuck table,comprising:

a first surface position detection means which has a first detectionlaser beam oscillation means for oscillating a first detection laserbeam having a predetermined wavelength to a first optical path anddetects the height position of an illuminated portion of the workpiecebased on the reflected light of the first detection laser beam;

a second surface position detection means which has a second detectionlaser beam oscillation means for oscillating a second detection laserbeam having a wavelength different from the wavelength of the firstdetection laser beam to a second optical path and detects the heightposition of an illuminated portion of the workpiece based on thereflected light of the second detection laser beam;

a synthetic beam splitter for guiding the first detection laser beamoscillated to the first optical path and the second detection laser beamoscillated to the second optical path to a third optical path;

a condenser which is installed in the third optical path and convergesthe first detection laser beam and the second detection laser beam toapply them to the workpiece held on the chuck table;

a focal point positioning means which is installed in the first orsecond optical path and changes the position of the focal point of thefirst detection laser beam or the second detection laser beam; and

a control means for obtaining the depth of a via hole formed in theworkpiece based on a detection value obtained by the first surfaceposition detection means and a detection value obtained by the secondsurface position detection means.

The above first surface position detection means comprises a firstnon-deflection beam splitter which is installed in the first opticalpath and guides light reflected from the workpiece to a fourth opticalpath, a pass filter which is installed in the fourth optical path andtransmits only reflected light having a wavelength corresponding to thatof the first detection laser beam out of the reflected light split bythe first non-deflection beam splitter, a non-deflection auxiliary beamsplitter for splitting the reflected light passing through the passfilter into a fifth optical path and a sixth optical path, a first lightreceiving device which receives 100% of the reflected light split intothe fifth optical path by the non-deflection auxiliary beam splitter, asecond light receiving device which receives the reflected light splitinto the sixth optical path by the non-deflection auxiliary beamsplitter, and a light receiving area restricting means which isinstalled in the sixth optical path and restricts the light receivingarea of the reflected light to be received by the second light receivingdevice;

the above second surface position detection means comprises a secondnon-deflection beam splitter which is installed in the second opticalpath and guides light reflected from the workpiece to a seventh opticalpath, a pass filter which is installed in the seventh optical path andtransmits only reflected light having a wavelength corresponding to thatof the second detection laser beam out of the reflected light split bythe second non-deflection beam splitter, a non-deflection auxiliary beamsplitter for splitting the reflected light passing through the passfilter into an eighth optical path and a ninth optical path, a firstlight receiving device which receives 100% of the reflected light splitinto the eighth optical path by the non-deflection auxiliary beamsplitter, a second light receiving device which receives the reflectedlight split into the ninth optical path by the non-deflection auxiliarybeam splitter, and a light receiving area restricting means which isinstalled in the ninth optical path and restricts the light receivingarea of the reflected light to be received by the second light receivingdevice; and

the above control means calculates the ratio of the quantity of lightreceived by the first light receiving device to the quantity of lightreceived by the second light receiving device of the first surfaceposition detection means based on detection signals from the first lightreceiving device and the second light receiving device to obtain theheight position of the top surface of the workpiece or the heightposition of the bottom surface of a via hole based on the ratio,calculates the ratio of the quantity of light received by the firstlight receiving device to the quantity of light received by the secondlight receiving device of the second surface position detection meansbased on detection signals from the first light receiving device and thesecond light receiving device to obtain the height position of thebottom surface of the via hole or the height position of the top surfaceof the workpiece based on the ratio, and obtains the depth of the viahole formed in the workpiece based on the height position of the topsurface of the workpiece and the height position of the bottom surfaceof the via hole.

The above control means comprises a memory having a storage area forstoring a control map showing the relationship between the ratio of thequantity of light received by the first light receiving device to thequantity of light received by the second light receiving device of thefirst surface position detection means or the second surface positiondetection means and the height position of a portion illuminated by thefirst detection laser beam or the second detection laser beam,calculates the ratio of the quantity of light received by the firstlight receiving device to the quantity of light received by the secondlight receiving device of the first surface position detection meansbased on detection signals from the first light receiving device and thesecond light receiving device to obtain the height position of the topsurface of the workpiece or the height position of the bottom surface ofthe via hole by collating the ratio with the control map, calculates theratio of the quantity of light received by the first light receivingdevice to the quantity of light received by the second light receivingdevice of the second surface position detection means based on detectionsignals from the first light receiving device and the second lightreceiving device to obtain the height position of the bottom surface ofthe via hole or the height position of the top surface of the workpieceby collating the ratio with the control map, and obtains the depth ofthe via hole formed in the workpiece based on the height position of thetop surface of the workpiece and the height position of the bottomsurface of the via hole.

The memory of the above control means has a storage area for storingdata on the reflectances of a plurality of substances, and the abovecontrol means judges whether the via hole formed in the workpiecereaches some other substance from the processed substance based on theamount of light received by the first light receiving device of thefirst surface position detection means or the second surface positiondetection means.

Further, according to the present invention, there is provided a viahole depth detector for detecting the depth of a via hole formed in aworkpiece held on the holding surface of a chuck table, comprising:

a detection laser beam oscillation means for oscillating a detectionlaser beam;

a deflection beam splitter for splitting the detection laser beamoscillated by the detection laser beam oscillator into a P wave and an Swave and guiding them to a first optical path and a second optical path,respectively;

a synthetic beam splitter for guiding the P wave and S wave of thedetection laser beam split by the deflection beam splitter to a thirdoptical path;

a condenser which is installed in the third optical path and convergesthe P wave and S wave of the detection laser beam to apply them to theworkpiece held on the chuck table;

a focal point positioning means which is installed in the first orsecond optical path and changes the focal point position of the P waveor S wave of the detection laser beam;

a first surface position detection means for detecting the heightposition of an illuminated portion of the workpiece based on thereflected light of the P wave or S wave of the detection laser beamapplied to the workpiece from the condenser;

a second surface position detection means for detecting the heightposition of an illuminated portion of the workpiece based on thereflected light of the S wave or P wave of the detection laser beamapplied to the workpiece from the condenser; and

a control means for obtaining the depth of a via hole formed in theworkpiece based on a detection value obtained by the first surfaceposition detection means and a detection value obtained by the secondsurface position detection means.

The above first surface position detection means comprises a firstnon-deflection beam splitter which is installed in the first opticalpath and guides light reflected from the workpiece to a fourth opticalpath, a pass filter which is installed in the fourth optical path andtransmits only the P wave or S wave of the detection laser beam out ofthe reflected light guided by the first non-deflection beam splitter, anon-deflection auxiliary beam splitter for splitting the reflected lightpassing through the pass filter into a fifth optical path and a sixthoptical path, a first light receiving device which receives 100% of thereflected light split into the fifth optical path by the non-deflectionauxiliary beam splitter, a second light receiving device which receivesthe reflected light split into the sixth optical path by thenon-deflection auxiliary beam splitter, and a light receiving arearestricting means which is installed in the sixth optical path andrestricts the light receiving area of the reflected light to be receivedby the second light receiving device;

the above second surface position detection means comprises a secondnon-deflection beam splitter which is installed in the second opticalpath and guides light reflected from the workpiece to a seventh opticalpath, a pass filter which is installed in the seventh optical path andtransmits only the S wave or P wave of the detection laser beam out ofthe reflected light guided by the second non-deflection beam splitter, anon-deflection auxiliary beam splitter for splitting the reflected lightpassing through the pass filter into an eighth optical path and a ninthoptical path, a first light receiving device which receives 100% of thereflected light split into the eighth optical path by the non-deflectionauxiliary beam splitter, a second light receiving device which receivesthe reflected light split into the ninth optical path by thenon-deflection auxiliary beam splitter, and a light receiving arearestricting means which is installed in the ninth optical path andrestricts the light receiving area of the reflected light to be receivedby the second light receiving device; and

the above control means calculates the ratio of the quantity of lightreceived by the first light receiving device to the quantity of lightreceived by the second light receiving device of the first surfaceposition detection means based on detection signals from the first lightreceiving device and the second light receiving device to obtain theheight position of the top surface of the workpiece or the heightposition of the bottom surface of a via hole based on the ratio,calculates the ratio of the quantity of light received by the firstlight receiving device to the quantity of light received by the secondlight receiving device of the second surface position detection meansbased on detection signals from the first light receiving device and thesecond light receiving device to obtain the height position of thebottom surface of the via hole or the height position of the top surfaceof the workpiece based on the ratio, and obtains the depth of the viahole formed in the workpiece based on the height position of the topsurface of the workpiece and the height position of the bottom surfaceof the via hole.

The above control means comprises a memory having a storage area forstoring a control map indicating the relationship between the ratio ofthe quantity of light received by the first light receiving device tothe quantity of light received by the second light receiving device ofthe first surface position detection means or the second surfaceposition detection means and the height position of a portionilluminated by the P wave or S wave of the detection laser beam,calculates the ratio of the quantity of light received by the firstlight receiving device to the quantity of light received by the secondlight receiving device of the first surface position detection meansbased on detection signals from the first light receiving device and thesecond light receiving device to obtain the height position of the topsurface of the workpiece or the height position of the bottom surface ofthe via hole by collating the ratio with the control map, calculates theratio of the quantity of light received by the first light receivingdevice to the quantity of light received by the second light receivingdevice of the second surface position detection means based on detectionsignals from the first light receiving device and the second lightreceiving device to obtain the height position of the bottom surface ofthe via hole or the height position of the top surface of the workpieceby collating the ratio with the control map, and obtains the depth ofthe via hole formed in the workpiece based on the height position of thetop surface of the workpiece and the height position of the bottomsurface of the via hole.

The memory of the above control means has a storage area for storingdata on the reflectances of a plurality of substances, and the controlmeans judges whether the via hole formed in the workpiece reachesanother substance from the processed substance based on the quantity oflight received by the first light receiving device of the first surfaceposition detection means or the second surface position detection means.

Since the via hole depth detector of the present invention obtains thedepth of a via hole formed in the workpiece based on a detection valueobtained by the first surface position detection means which comprisesthe first detection laser beam oscillation means and detects the heightposition of an illuminated portion of the workpiece based on thereflected light of the first detection laser beam and a detection valueobtained by the second surface position detection means which comprisesthe second detection laser beam oscillation means and detects the heightposition of an illuminated portion of the workpiece based on thereflected light of the second detection laser beam. Therefore, the depthof the via hole formed in the workpiece can be detected efficiently.Consequently, an incomplete via hole can be re-processed without fail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a laser beam processing machinecomprising a via hole depth detector constituted according to thepresent invention;

FIG. 2 is a schematic block diagram of a laser beam application meansand a via hole depth detector constituted according to the presentinvention provided in the laser beam processing machine shown in FIG. 1;

FIG. 3 is an explanatory diagram showing a state where the illuminatedarea changes according to the application position of a first detectionlaser beam LB1 illuminated from the first surface position detectionmeans of the via hole depth detector shown in FIG. 2;

FIG. 4 is a control map showing the relationship between the ratio of avoltage value (V1) output from the first light receiving device to avoltage value (V2) output from the second light receiving device of thevia hole depth detector shown in FIG. 2 and the height from anappropriate position to the bottom surface of a via hole;

FIG. 5 is an explanatory diagram showing a state where the illuminatedarea changes according to the application position of a second detectionlaser beam LB2 illuminated from the second surface position detectionmeans of the via hole depth detector shown in FIG. 2;

FIG. 6 is a plan view of a semiconductor wafer as a workpiece;

FIG. 7 is a partially enlarged plan view of the semiconductor wafershown in FIG. 6;

FIG. 8 is a perspective view of the semiconductor wafer shown in FIG. 6which is put on the surface of a protective tape mounted on an annularframe;

FIG. 9 is an explanatory diagram showing the relationship between thesemiconductor wafer shown in FIG. 6 and its coordinate position in astate where it is held at a predetermined position of the chuck table ofthe laser beam processing machine shown in FIG. 1;

FIG. 10 is an explanatory diagram showing a laser processing step forforming a laser-processed via hole in the semiconductor wafer shown inFIG. 6 by the laser beam processing machine shown in FIG. 1;

FIG. 11 is an enlarged sectional view of the principal portion of thesemiconductor wafer in which laser-processed via holes have been formedby the laser processing step shown in FIG. 10;

FIGS. 12( a) and 12(b) are explanatory diagrams showing the via holedepth detection step which is carried out by the via hole depth detectorprovided in the laser beam processing machine shown in FIG. 1;

FIG. 13 is a diagram showing an example of a decision map prepared bythe control means provided in the laser beam processing machine shown inFIG. 1; and

FIG. 14 is a block diagram showing another embodiment of the via holedepth detector constituted according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described indetail hereinbelow with reference to the accompanying drawings.

FIG. 1 is a perspective view of a laser beam processing machinecomprising a via hole depth detector constituted according to thepresent invention. The laser beam processing machine 1 shown in FIG. 1comprises a stationary base 2, a chuck table mechanism 3 for holding aworkpiece, which is mounted on the stationary base 2 in such a mannerthat it can move in a processing-feed direction (X direction) indicatedby an arrow X, a laser beam application unit support mechanism 4 mountedon the stationary base 2 in such a manner that it can move in anindexing-feed direction (Y direction) indicated by an arrow Yperpendicular to the direction indicated by the arrow X, and a laserbeam application unit 5 mounted to the laser beam application unitsupport mechanism 4 in such a manner that it can move in a direction (Zdirection) indicated by an arrow Z.

The above chuck table mechanism 3 comprises a pair of guide rails 31 and31 mounted on the stationary base 2 and arranged parallel to each otherin the processing-feed direction indicated by the arrow X, a firstsliding block 32 mounted on the guide rails 31 and 31 in such a mannerthat it can move in the processing-feed direction indicated by the arrowX, a second sliding block 33 mounted on the first sliding block 32 insuch a manner that it can move in the indexing-feed direction indicatedby the arrow Y, a support table 35 supported on the second sliding block33 by a cylindrical member 34, and a chuck table 36 as a workpieceholding means. This chuck table 36 comprises an adsorption chuck 361made of a porous material, and a workpiece, for example, a disk-likesemiconductor wafer is held on a holding surface which is the topsurface of the adsorption chuck 361 by a suction means that is notshown. The chuck table 36 constituted as described above is turned by apulse motor (not shown) installed in the cylindrical member 34. Thechuck table 36 is provided with clamps 362 for fixing an annular framewhich will be described later.

The above first sliding block 32 has, on its undersurface, a pair ofto-be-guided grooves 321 and 321 to be fitted to the above pair of guiderails 31 and 31 and, on the top surface, a pair of guide rails 322 and322 formed parallel to each other in the indexing-feed directionindicated by the arrow Y. The first sliding block 32 constituted asdescribed above can move along the pair of guide rails 31 and 31 in theprocessing-feed direction indicated by the arrow X by fitting theto-be-guided grooves 321 and 321 to the pair of guide rails 31 and 31,respectively. The chuck table mechanism 3 in the illustrated embodimentcomprises a processing-feed means 37 as an X-axis moving means formoving the first sliding block 32 along the pair of guide rails 31 and31 in the processing-feed direction indicated by the arrow X. Theprocessing-feed means 37 comprises a male screw rod 371 arranged betweenthe above pair of guide rails 31 and 31 in parallel thereto, and a drivesource such as a pulse motor 372 for rotary-driving the male screw rod371. The male screw rod 371 is, at its one end, rotatably supported to abearing block 373 fixed on the above stationary base 2 and is, at theother end, transmission-coupled to the output shaft of the above pulsemotor 372. The male screw rod 371 is screwed into a threadedthrough-hole formed in an unshown female screw block projecting from theundersurface of the center portion of the first sliding block 32.Therefore, by driving the male screw rod 371 in a normal direction oradverse direction with the pulse motor 372, the first sliding block 32is moved along the guide rails 31 and 31 in the processing-feeddirection indicated by the arrow X.

The laser beam processing machine 1 in the illustrated embodimentcomprises a processing-feed amount detection means 374 for detecting theprocessing-feed amount of the above chuck table 36. The processing-feedamount detection means 374 is composed of a linear scale 374 a arrangedalong the guide rail 31 and a read head 374 b which is mounted on thefirst sliding block 32 and moves along the linear scale 374 a togetherwith the first sliding block 32. The read head 374 b of thisprocessing-feed amount detection means 374 supplies one pulse signal forevery 1 μm to a control means which will be described later in theillustrated embodiment. The control means later described counts theinput pulse signals to detect the processing-feed amount of the chucktable 36. When the pulse motor 372 is used as a drive source for theabove processing-feed means 37, the processing-feed amount of the chucktable 36 can be detected by counting the drive pulses of the controlmeans later described for outputting a drive signal to the pulse motor372. When a servo motor is used as a drive source for the aboveprocessing-feed means 37, pulse signals outputted from a rotary encoderfor detecting the revolution of the servo motor are supplied into thecontrol means later-described, and the control means counts the pulsesignals input, thereby making it possible to detect the processing-feedamount of the chuck table 36.

The above second sliding block 33 has, on the undersurface, a pair ofto-be-guided grooves 331 and 331 to be fitted to the pair of guide rails322 and 322 on the top surface of the above first sliding block 32 andcan move in the indexing-feed direction indicated by the arrow Y byfitting the to-be-guided grooves 331 and 331 to the pair of guide rails322 and 322, respectively. The chuck table mechanism 3 in theillustrated embodiment comprises a first indexing-feed means 38 as afirst Y-axis moving means for moving the second sliding block 33 alongthe pair of guide rails 322 and 322 on the first sliding block 32 in theindexing-feed direction indicated by the arrow Y. The firstindexing-feed means 38 comprises a male screw rod 381 which is arrangedbetween the above pair of guide rails 322 and 322 in parallel thereto,and a drive source such as a pulse motor 382 for rotary-driving the malescrew rod 381. The male screw rod 381 is, at its one end, rotatablysupported to a bearing block 383 fixed on the top surface of the abovefirst sliding block 32 and is, at the other end, transmission coupled tothe output shaft of the above pulse motor 382. The male screw rod 381 isscrewed into a threaded through-hole formed in a female screw block (notshown) projecting from the undersurface of the center portion of thesecond sliding block 33. Therefore, by driving the male screw rod 381 ina normal direction or adverse direction with the pulse motor 382, thesecond sliding block 33 is moved along the guide rails 322 and 322 inthe indexing-feed direction indicated by the arrow Y.

The laser beam processing machine 1 in the illustrated embodimentcomprises an indexing-feed amount detection means 384 for detecting theindexing-feed amount of the above second sliding block 33. Thisindexing-feed amount detection means 384 comprises a linear scale 384 aarranged along the guide rail 322 and a read head 384 b which is mountedon the second sliding block 33 and moves along the linear scale 384 atogether with the second sliding block 33. The read head 384 b of theindexing-feed amount detection means 384 supplies one pulse signal forevery 1 μm to the control means later described in the illustratedembodiment. The control means later described counts the input pulsesignals to detect the indexing-feed amount of the chuck table 36. Whenthe pulse motor 382 is used as a drive source for the above firstindexing-feed means 38, the indexing-feed amount of the chuck table 36can be detected by counting the drive pulses of the control means laterdescribed for outputting a drive signal to the pulse motor 382. When aservo motor is used as a drive source for the above first indexing-feedmeans 38, pulse signals outputted from a rotary encoder for detectingthe revolution of the servo motor are supplied into the control meanslater-described, and the control means counts the pulse signals input,thereby making it possible to detect the indexing-feed amount of thechuck table 36.

The above laser beam application unit support mechanism 4 comprises apair of guide rails 41 and 41 mounted on the stationary base 2 andarranged parallel to each other in the indexing-feed direction indicatedby the arrow Y and a movable support base 42 mounted on the guide rails41 and 41 in such a manner that it can move in the direction indicatedby the arrow Y. This movable support base 42 consists of a movablesupport portion 421 movably mounted on the guide rails 41 and 41 and amounting portion 422 mounted on the movable support portion 421. Themounting portion 422 is provided with a pair of guide rails 423 and 423extending parallel to each other in the direction indicated by the arrowZ on one of its flanks. The laser beam application unit supportmechanism 4 in the illustrated embodiment comprises a secondindexing-feed means 43 as a second Y-axis moving means for moving themovable support base 42 along the pair of guide rails 41 and 41 in theindexing-feed direction indicated by the arrow Y. This secondindexing-feed means 43 comprises a male screw rod 431 arranged betweenthe above pair of guide rails 41 and 41 in parallel thereto and a drivesource such as a pulse motor 432 for rotary-driving the male screw rod431. The male screw rod 431 is, at its one end, rotatably supported to abearing block (not shown) fixed on the above stationary base 2 and is,at the other end, transmission-coupled to the output shaft of the abovepulse motor 432. The male screw rod 431 is screwed into a threadedthrough-hole formed in a female screw block (not shown) projecting fromthe undersurface of the center portion of the movable support portion421 constituting the movable support base 42. Therefore, by driving themale screw rod 431 in a normal direction or adverse direction with thepulse motor 432, the movable support base 42 is moved along the guiderails 41 and 41 in the indexing-feed direction indicated by the arrow Y.

The laser beam application unit 5 in the illustrated embodimentcomprises a unit holder 51 and a laser beam application means 52 securedto the unit holder 51. The unit holder 51 has a pair of to-be-guidedgrooves 511 and 511 to be slidably fitted to the pair of guide rails 423and 423 on the above mounting portion 422 and is supported in such amanner that it can move in the direction (Z direction) indicated by thearrow Z by fitting the to-be-guided grooves 511 and 511 to the aboveguide rails 423 and 423, respectively.

The illustrated laser beam application means 52 has a cylindrical casing521 extending substantially horizontally. In the casing 521, there areinstalled a processing pulse laser beam oscillation means 6 and adirection changing mirror 7 for changing the direction of a processingpulse laser beam oscillated from this processing pulse laser beamoscillation means 6 at 90° to a downward direction in FIG. 2. Acondenser 8 comprising a condenser lens 81 for converging a laser beamwhose direction has been changed by the direction changing mirror 7 ismounted on the end of the casing 521 (see FIG. 1). The processing pulselaser beam oscillation means 6 oscillates a processing pulse laser beamLB of a wavelength having absorptivity for a wafer as the workpiece. Asthis processing pulse laser beam oscillation means 6 may be used a YVO4pulse laser oscillator or a YAG pulse laser oscillator which oscillatesthe processing pulse laser beam LB having a wavelength of, for example,355 nm when the wafer comprises a silicon substrate, silicon carbidesubstrate, lithium tantalate substrate, glass substrate or quartzsubstrate.

The condenser 8 comprising the above condenser lens 81 is mounted ontothe end of the above casing 521. This condenser 8 is constituted by acombination of lenses including the condenser lens 81, and focuses theprocessing pulse laser beam LB which has been oscillated from the aboveprocessing pulse laser beam oscillation means 6 and whose direction hasbeen changed by the direction changing mirror 7, at a focal point P.

Still referring to FIG. 2, the laser beam processing machine 1 in theillustrated embodiment has a detector 9 for detecting the depth of a viahole formed in the workpiece held on the chuck table 36. In theillustrated embodiment, the via hole depth detector 9 is composed of afirst surface position detection means 9 a having a first detectionlaser beam oscillation means 90 a for oscillating a first detectionlaser beam LB1 to a first optical path A and a second surface positiondetection means 9 b having a second detection laser beam oscillationmeans 90 b for oscillating a second detection laser beam LB2 to a secondoptical path B. Further, the via hole depth detector 9 comprises asynthetic beam splitter 91 for guiding the first detection laser beamLB1 oscillated to the first optical path A and the second detectionlaser beam LB2 oscillated to the second optical path B, to a thirdoptical path C. The above condenser 8 is designed to be installed in thethird optical path C.

The above first surface position detection means 9 a comprises the abovefirst detection laser beam oscillation means 90 a, a dichroic halfmirror 91 a interposed between the above processing pulse laser beamoscillation means 6 and the synthetic beam splitter 91, and a firstnon-deflection beam splitter 92 a interposed between the dichroic halfmirror 91 a and the first detection laser beam oscillation means 90 a.As the first detection laser beam oscillation means 90 a may be used aHe—Ne laser oscillator which oscillates the first detection laser beamLB1 having a wavelength of, for example, 633 nm. The dichroic halfmirror 91 a transmits the processing pulse laser beam LB oscillated fromthe processing pulse laser beam oscillation means 6 toward the syntheticbeam splitter 91 and changes the direction of the first detection laserbeam LB1 oscillated from the first detection laser beam oscillationmeans 90 a toward the synthetic beam splitter 91. The firstnon-deflection beam splitter 92 a transmits the first detection laserbeam LB1 and changes the direction of light reflected by the dichroichalf mirror 91 a to a fourth optical path D.

The first surface position detection means 9 a in the illustratedembodiment further comprises a pass filter 93 a which transmits onlyreflected light corresponding to the wavelength (for example, 633 nm) ofthe first detection laser beam LB1 out of light reflected by the firstnon-deflection beam splitter 92 a, a non-deflection auxiliary beamsplitter 94 a for splitting the reflected light passing through the passfilter 93 a into a fifth optical path E and a sixth optical path F, acondenser lens 95 a for converging 100% of the reflected light splitinto the fifth optical path E by the non-deflection auxiliary beamsplitter 94 a, and a first light receiving device 96 a for receiving thereflected light converged by the condenser lens 95 a. The first lightreceiving device 96 a supplies a voltage signal corresponding to thequantity of its received light to the control means that will bedescribed later. The first surface position detection means 9 a in theillustrated embodiment further comprises a second light receiving device97 a for receiving the reflected light split into the sixth optical pathF by the non-deflection auxiliary beam splitter 94 a, and a lightreceiving area restricting means 98 a for restricting the receiving areaof the reflected light to be received by the second light receivingdevice 97 a. The light receiving area restricting means 98 a in theillustrated embodiment is composed of a cylindrical lens 981 a forconverging the reflected light split into the sixth optical path F bythe non-deflection auxiliary beam splitter 94 a unidimensionally and aunidimensional mask 982 a for restricting the reflected light convergedunidimensionally by the cylindrical lens 981 a to a unit length. Thesecond light receiving device 97 a for receiving the reflected lightpassing through the unidimensional mask 982 a supplies a voltage signalcorresponding to the quantity of its received light to the control meansthat will be described later.

A description will be subsequently given of the second surface positiondetection means 9 b constituting the via hole depth detector 9.

The second surface position detection means 9 b has the above seconddetection laser beam oscillation means 90 b for oscillating the seconddetection laser beam LB2, and a second non-deflection beam splitter 92 binterposed between the second detection laser beam oscillation means 90b and the above synthetic beam splitter 91. As the second detectionlaser beam oscillation means 90 b may be used a YAG laser oscillatorwhich oscillates the second detection laser beam LB2 having a wavelengthof, for example, 532 nm. The second non-deflection beam splitter 92 btransmits the second detection laser beam LB2 and changes the directionof light reflected by the above synthetic beam splitter 91, to a seventhoptical path G.

The second surface position detection means 9 b in the illustratedembodiment further comprises a pass filter 93 b which transmits onlyreflected light corresponding to the wavelength (for example, 532 nm) ofthe second detection laser beam LB2 out of light reflected by the secondnon-deflection beam splitter 92 b, a non-deflection auxiliary beamsplitter 94 b for splitting the reflected light passing through the passfilter 93 b into an eighth optical path J and a ninth optical path K, acondenser lens 95 b for converging 100% of the reflected light splitinto the eighth optical path J by the non-deflection auxiliary beamsplitter 94 b, and a first light receiving device 96 b for receiving thereflected light converged by the condenser lens 95 b. The first lightreceiving device 96 b supplies a voltage signal corresponding to thequantity of its received light to the control means that is describedlater. The second surface position detection means 9 b in theillustrated embodiment further comprises a second light receiving device97 b for receiving the reflected light split into the ninth optical pathK by the non-deflection auxiliary beam splitter 94 b, and a lightreceiving area restricting means 98 b for restricting the receiving areaof the reflected light to be received by the second light receivingdevice 97 b. The light receiving area restricting means 98 b in theillustrated embodiment is composed of a cylindrical lens 981 b forconverging the reflected light split into the ninth optical path K bythe non-deflection auxiliary beam splitter 94 b unidimensionally and aunidimensional mask 982 b for restricting the reflected light convergedunidimensionally by the cylindrical lens 981 b to a unit length. Thesecond light receiving device 97 b for receiving the reflected lightpassing through the unidimensional mask 982 b supplies a voltage signalcorresponding to the quantity of its received light to the control meansthat is described later. The second surface position detection means 9 bin the illustrated embodiment comprises a beam expander 99 as a focalpoint positioning means interposed between the above synthetic beamsplitter 91 and the second non-deflection beam splitter 92 b. This beamexpander 99 is composed of two convex lenses 991 and 992 and anadjusting means (not shown) for adjusting the interval between the twoconvex lenses 991 and 992.

The via hole depth detector 9 in the illustrated embodiment isconstituted as described above, and its function will be describedhereinbelow. A description is first given of the function of the firstsurface position detection means 9 a constituting the via hole depthdetector 9.

The first detection laser beam LB1 oscillated from the first detectionlaser beam oscillation means 90 a of the first surface positiondetection means 9 a passes through the first non-deflection beamsplitter 92 a and reaches the dichroic half mirror 91 a to be reflectedtoward the synthetic beam splitter 91. The first detection laser beamLB1 reflected toward the synthetic beam splitter 91 passes through thesynthetic beam splitter 91 and the third optical path C, is deflected bythe direction changing mirror 7 and converged by the condenser lens 81,like the above processing pulse laser beam LB. The first detection laserbeam LB1 converged as described above is reflected on the front surface(top surface) of the workpiece held on the chuck table 36 and itsreflected light reaches the pass filter 93 a through the condenser lens81, the direction changing mirror 7, the synthetic beam splitter 91, thedichroic half mirror 91 a and the first non-deflection beam splitter 92a, as shown by a broken line in FIG. 2. As will be described later, thereflected light of the second detection laser beam LB2 also reaches thepass filter 93 a through the same route as the first detection laserbeam LB1. Since the pass filter 93 a is so constituted as to transmitonly reflected light corresponding to the wavelength (for example, 633nm) of the first detection laser beam LB1 as described above, thereflected light of the second detection laser beam LB2 is cut off by thepass filter 93 a. Therefore, only the reflected light of the firstdetection laser beam LB1 passes through the pass filter 93 a and reachesthe non-deflection auxiliary beam splitter 94 a.

The reflected light of the first detection laser beam LB1 which hasreached the non-deflection auxiliary beam splitter 94 a is split intothe fifth optical path E and the sixth optical path F. The reflectedlight split into the fifth optical path E is 100% converged by thecondenser lens 95 a and received by the first light receiving device 96a. The first light receiving device 96 a supplies then a voltage signalcorresponding to the quantity of its received light to the control meansthat is described later. Meanwhile, the reflected light of the firstdetection laser beam LB1 split into the sixth optical path F isconverged unidimensionally by the cylindrical lens 981 a of the lightreceiving area restricting means 98 a, restricted to a predeterminedunit length by the unidimensional mask 982 a and received by the secondlight receiving device 97 a. Then, the second light receiving device 97a supplies a voltage signal corresponding to the quantity of itsreceived light to the control means that is described later.

A description will be subsequently given of the quantity of thereflected light of the first detection laser beam LB1 to be received bythe first light receiving device 96 a and the second light receivingdevice 97 a. Since the reflected light of the first detection laser beamLB1 to be received by the first light receiving device 96 a is 100%converged by the condenser lens 95 a, the quantity of received light isconstant and the voltage value (V1) output from the first lightreceiving device 96 a is constant (for example, 10 V). Meanwhile, thereflected light of the first detection laser beam LB1 to be received bythe second light receiving device 97 a is converged unidimensionally bythe cylindrical lens 981 a, then restricted to the predetermined unitlength by the unidimensional mask 982 a and received by the second lightreceiving device 97 a. Therefore, the quantity of light received by thesecond light receiving device 97 a changes according to the position ofthe focal point P1 of the first detection laser beam LB1 focused by thecondenser lens 81 of the condenser 8 in the workpiece. Accordingly, thevoltage value (V2) output from the second light receiving device 97 achanges according to the position of the focal point P1 of the firstdetection laser beam LB1 in the workpiece.

For instance, when the first detection laser beam LB1 is applied to theworkpiece W held on the chuck table 36 by setting its focal point P1 toa position at a height h1 above the surface (top surface) of the chucktable 36, the first detection laser beam LB1 is reflected at an area S1on the front surface (top surface) of the workpiece W. This reflectedlight is split into the fifth optical path E and the sixth optical pathF by the non-deflection auxiliary beam splitter 94 a as described above.Since the reflected light of the area S1 split into the fifth opticalpath E is 100% converged by the condenser lens 95 a, all the quantity ofthe reflected light is received by the first light receiving device 96a. Meanwhile, since the reflected light of the area S1 split into thesixth optical path F by the non-deflection auxiliary beam splitter 94 ais converged unidimensionally by the cylindrical lens 981 a, its sectionbecomes elliptic. Therefore, as the reflected light having an ellipticsection is restricted to the predetermined unit length by theunidimensional mask 982 a, part of the reflected light split into thesixth optical path F is received by the second light receiving device 97a. Consequently, the quantity of light received by the second lightreceiving device 97 a becomes smaller than that of the first lightreceiving device 96 a. Thus, the reflected light converged to have anelliptic section is restricted to the predetermined unit length by theunidimensional mask 982 a and part of the reflected light is received bythe second light receiving device 97 a. Accordingly, the quantity of thereflected light received by the second light receiving device 97 abecomes smaller as the front surface (top surface) of the workpiece Wbecomes higher above the focal point P1 of the first detection laserbeam LB1.

The relationship between the ratio of the voltage value (V1) output fromthe above first light receiving device 96 a to the voltage value (V2)output from the second light receiving device 97 a and the heightposition of the front surface (top surface) of the workpiece W to whichthe first detection laser beam LB1 is applied will be described withreference to a control map shown in FIG. 4. In FIG. 4, the horizontalaxis shows the height from the focal point P1 and the vertical axisshows the ratio (V1/V2) of the voltage value (V1) output from the firstlight receiving device 96 a to the voltage value (V2) output from thesecond light receiving device 97 a.

In the example shown in FIG. 4, when the above voltage value ratio(V1/V2) is “1”, the focal point P1 of the first detection laser beam LB1is existent at the front surface (top surface) of the workpiece W. Asthe height from the focal point P1 to the front surface (top surface) ofthe workpiece W becomes larger, the above voltage value ratio (V1/V2)becomes higher. Therefore, the height hx from the focal point P1 to thefront surface (top surface) of the workpiece W can be detected byobtaining the above voltage value ratio (V1/V2). The thickness H1 of theworkpiece W can be obtained by adding the height hx from the focal pointP1 to the height h1 from the surface (top surface) of the chuck table 36to the focal point P1 (H1=h1+hx). The control map shown in FIG. 4 isstored in the memory of the control means that is described later.

A description will be subsequently given of the function of the secondsurface position detection means 9 b constituting the via hole depthdetector 9.

The second detection laser beam LB2 oscillated from the second detectionlaser beam oscillation means 90 b of the second surface positiondetection means 9 b passes through the second non-deflection beamsplitter 92 b, reaches the synthetic beam splitter 91 to be deflectedtoward the third optical path C and is converged by the condenser lens81 like the above processing pulse laser beam LB and the first detectionlaser beam LB1. The second detection laser beam LB2 converged asdescribed above is reflected on the front surface (top surface) of theworkpiece held on the chuck table 36, and its reflected light reachesthe pass filter 93 b through the condenser lens 81, the directionchanging mirror 7, the synthetic beam splitter 91 and the secondnon-deflection beam splitter 92 b as shown by a broken line in FIG. 2.The reflected light of the first detection laser beam LB1 also reachesthe pass filter 93 b through the same route as the second detectionlaser beam LB2. Since the pass filter 93 b is so constituted as totransmit only reflected light corresponding to the wavelength (forexample, 532 nm) of the second detection laser beam LB2 as describedabove, the reflected light of the first detection laser beam LB1 is cutoff by the pass filter 93 b. Therefore, only the reflected light of thesecond detection laser beam LB2 passes through the pass filter 93 b andreaches the non-deflection auxiliary beam splitter 94 b.

The reflected light of the second detection laser beam LB2 which hasreached the non-deflection auxiliary beam splitter 94 b is split intothe eighth optical path J and the ninth optical path K. The reflectedlight split into the eighth optical path J is 100% converged by thecondenser lens 95 b and received by the first light receiving device 96b. The first light receiving device 96 b then supplies a voltage signalcorresponding to the quantity of its received light to the control meansthat is described later. Meanwhile, the reflected light of the seconddetection laser beam LB2 split into the ninth optical path K isconverged unidimensionally by the cylindrical lens 981 b, restricted toa predetermined unit length by the unidimensional mask 982 b of thelight receiving area restricting means 98 b and received by the secondlight receiving device 97 b. Then, the second light receiving device 97b supplies a voltage signal corresponding to the quantity of itsreceived light to the control means that is described later.

A description will be subsequently given of the quantity of thereflected light of the second detection laser beam LB2 to be received bythe first light receiving device 96 b and the second light receivingdevice 97 b. Since the reflected light of the second detection laserbeam LB2 to be received by the first light receiving device 96 b is 100%converged by the condenser lens 95 b, the quantity of received light isconstant and the voltage value (V1) output from the first lightreceiving device 96 b is constant (for example, 10 V). Meanwhile, thereflected light of the second detection laser beam LB2 to be received bythe second light receiving device 97 b is converged unidimensionally bythe cylindrical lens 981 b, then restricted to the predetermined unitlength by the unidimensional mask 982 b and received by the second lightreceiving device 97 b. Therefore, the quantity of light received by thesecond light receiving device 97 b changes according to the position ofthe focal point P2 of the second detection laser beam LB2 focused by thecondenser lens 81 of the condenser 8 in the workpiece. Accordingly, thevoltage value (V2) output from the second light receiving device 97 bchanges according to the position of the focal point P2 of the seconddetection laser beam LB2 in the workpiece.

For instance, when the second detection laser beam LB2 is to be appliedto the bottom surface of the via hole H formed in the workpiece W heldon the chuck table 36, the focal point P2 is set to the top surface ofthe chuck table 36, that is, the undersurface of the workpiece W asshown in FIG. 5. The positioning of the focal point P2 of the seconddetection laser beam LB2 is carried out by controlling the intervalbetween the two convex lenses 991 and 992 constituting the beam expander99 as the focal point positioning means. When the second detection laserbeam LB2 is applied by setting its focal point P2 to the undersurface ofthe workpiece W, the second detection laser beam LB2 is reflected at anarea S2 on the bottom surface of the via hole H formed in the workpieceW. This reflected light is split into the eighth optical path J and theninth optical path K by the non-deflection auxiliary beam splitter 94 bas described above. Since the reflected light of the area S2 split intothe eighth optical path J is 100% converged by the condenser lens 95 b,all the quantity of the reflected light is received by the first lightreceiving device 96 b. Meanwhile, since the reflected light of the areaS2 split into the ninth optical path K by the non-deflection auxiliarybeam splitter 94 b is converged unidimensionally by the cylindrical lens981 b, its section becomes elliptic. Therefore, as the reflected lighthaving an elliptic section is restricted to the predetermined unitlength by the unidimensional mask 982 b, part of the reflected lightsplit into the ninth optical path K is received by the second lightreceiving device 97 b. Accordingly, the quantity of light received bythe second light receiving device 97 b becomes smaller than that of thefirst light receiving device 96 b. Thus, the reflected light convergedto have an elliptic section is restricted to the predetermined length bythe unidimensional mask 982 b, and part of the reflected light isreceived by the second light receiving device 97 b. Therefore, thequantity of the reflected light received by the second light receivingdevice 97 b becomes smaller as the bottom surface of the via hole Hbecomes higher above the focal point P2 of the second detection laserbeam LB2.

The relationship between the ratio of the voltage value (V1) output fromthe first light receiving device 96 b to the voltage value (V2) outputfrom the second light receiving device 97 b and the bottom surface ofthe via hole formed in the workpiece W to which the second detectionlaser beam LB2 is applied will be described with reference to thecontrol map shown in FIG. 4. The height position of the bottom surfaceof the via hole H formed in the workpiece W is obtained with referenceto the control map shown in FIG. 4 as follows. When the above voltagevalue ratio (V1/V2) is “1”, the bottom surface of the via hole H reachesthe undersurface of the workpiece W and the height T from the focalpoint P2 becomes nil (0). As the height T from the focal point P2 to thebottom surface of the via hole H formed in the workpiece W becomeslarger, the above voltage value ratio (V1/V2) becomes higher as shown inFIG. 4. Therefore, the height T from the focal point P2 to the bottomsurface of the via hole formed in the workpiece W can be detected byobtaining the above voltage value ratio (V1/V2). Thus, by setting thefocal point P2 of the second detection laser beam LB2 to theundersurface of the workpiece W, the above height T becomes the heightfrom the undersurface of the workpiece W to the bottom surface of thevia hole H. Therefore, the depth H2 of the via hole H formed in theworkpiece W can be obtained by subtracting the height T from theundersurface of the workpiece W to the bottom surface of the via hole Hformed in the workpiece W detected by the second surface positiondetection means 9 b from the thickness H1 of the workpiece W detected bythe above first surface position detection means 9 a (H2=H1−T).

Returning to FIG. 1, an image pick-up means 11 for detecting the area tobe processed by the laser beam application means 52 is mounted onto theend portion of the casing 521 constituting the above laser beamapplication means 52. This image pick-up means 11 is constituted by aninfrared illuminating means for applying infrared radiation to theworkpiece, an optical system for capturing infrared radiation irradiatedby the infrared illuminating means, and an image pick-up device(infrared CCD) for outputting an electric signal corresponding toinfrared radiation captured by the optical system, in addition to anordinary image pick-up device (CCD) for picking up an image with visibleradiation. An image signal picked-up is supplied to the control meansthat is described later.

The laser beam application unit 5 in the illustrated embodiment has afocal point positioning means 53 as a Z-axis moving means for moving theunit holder 51 along the pair of guide rails 423 and 423 in thedirection indicated by the arrow Z (direction perpendicular to theholding surface which is the top surface of the adsorption chuck 361).The focal point positioning means 53 comprises a male screw rod (notshown) arranged between the pair of guide rails 423 and 423 and a drivesource such as a pulse motor 532 for rotary-driving the male screw rod.By driving the male screw rod (not shown) in a normal direction oradverse direction with the pulse motor 532, the unit holder 51 and thelaser beam application means 52 comprising the condenser 8 are movedalong the guide rails 423 and 423 in the direction indicated by thearrow Z. In the illustrated embodiment, the laser beam application means52 is moved up by driving the pulse motor 532 in a normal direction andmoved down by driving the pulse motor 532 in the adverse direction.

The laser beam processing machine 1 in the illustrated embodimentcomprises the control means 10. The control means 10 is composed of acomputer which comprises a central processing unit (CPU) 101 forcarrying out arithmetic processing based on a control program, aread-only memory (ROM) 102 for storing the control program, etc., aread/write random access memory (RAM) 103 for storing the results ofoperations, a counter 104, an input interface 105 and an outputinterface 106. Detection signals from the above processing-feed amountdetection means 374, the indexing-feed amount detection means 384, thefirst light receiving device 96 a and the second light receiving device97 a of the first surface position detection means 9 a, the first lightreceiving device 96 b and the second light receiving device 97 b of thesecond surface position detection means 9 b and the image pick-up means11 are inputted to the input interface 105 of the control means 10.Control signals are outputted to the above pulse motor 372, the pulsemotor 382, the pulse motor 432, the pulse motor 532, the processingpulse laser beam oscillation means 6, the first detection laser beamoscillation means 90 a, the second detection laser beam oscillationmeans 90 b and display means 100 from the output interface 106 of thecontrol means 10. The above random access memory (RAM) 103 has a firststorage area 103 a for storing the control map shown in FIG. 4, a secondstorage area 103 b for storing data on the design values of theworkpiece which will be described later, and other storage areas.

The laser beam processing machine 1 in the illustrated embodiment isconstituted as described above, and its function will be describedhereinbelow.

FIG. 6 is a plan view of a semiconductor wafer 30 as the workpiece to beprocessed by a laser beam. The semiconductor wafer 30 shown in FIG. 6is, for example, a silicon wafer having a thickness of 100 μm, aplurality of areas are sectioned by a plurality of dividing lines 301formed in a lattice pattern on the front surface 30 a, and a device 302such as IC or LSI is formed in each of the sectioned areas. The devices302 are all the same in constitution. A plurality of electrodes 303 (303a to 303 j) are formed on the surface of each device 302, as shown inFIG. 7. In the illustrated embodiment, electrodes 303 a and 303 f,electrodes 303 b and 303 g, electrodes 303 c and 303 h, electrodes 303 dand 303 i, and electrodes 303 e and 303 j are at the same positions inthe X direction. Via holes reaching the electrodes 303 from the rearsurface 30 b are formed in portions corresponding to the plurality ofelectrodes 303 (303 a to 303 j), respectively. The intervals A betweenadjacent electrodes 303 (303 a to 303 j) in the X direction (horizontaldirection in FIG. 7) and the intervals B between adjacent electrodes inthe X direction (horizontal direction in FIG. 7) with the dividing line301 interposed therebetween, for example, between the electrodes 303 eand 303 a out of the electrodes 303 formed on each device 302 are thesame in the illustrated embodiment. The intervals C between adjacentelectrodes 303 (303 a to 303 j) in the Y direction (vertical directionin FIG. 7) and the intervals D between adjacent electrodes in the Ydirection (vertical direction in FIG. 7) with the dividing line 301interposed therebetween, for example, between the electrodes 303 f and303 a and between the electrodes 303 j and 303 e out of the electrodes303 formed on each device 302 are the same in the illustratedembodiment. The design value data of the semiconductor wafer 30constituted as described above, which include the numbers of devices 302disposed in rows E1 to En and columns F1 to Fn shown in FIG. 6 and theabove intervals A, B, C and D, are stored in the second storage area 103b of the above random access memory (RAM) 103.

An example of laser processing for forming a via hole in portionscorresponding to the electrodes 303 (303 a to 303 j) of each device 302formed on the above semiconductor wafer 30 by using the above laser beamprocessing machine 1 will be described hereinunder.

The front surface 30 a of the semiconductor wafer 30 constituted asdescribed above is put on a protective tape 50 which is formed of asynthetic resin sheet such as a polyolefin sheet and is mounted on anannular frame 40 as shown in FIG. 8. Therefore, the rear surface 30 b ofthe semiconductor wafer 30 faces up. The protective tape 50 side of thesemiconductor wafer 30 supported to the annular frame 40 through theprotective tape 50 is placed on the chuck table 36 of the laser beamprocessing machine 1 shown in FIG. 1. The semiconductor wafer 30 issuction-held on the chuck table 36 through the protective tape 50 byactivating the suction means that is not shown. The annular frame 40 isfixed by the clamps 362.

The chuck table 36 suction-holding the semiconductor wafer 30 asdescribed above is brought to a position right below the image pick-upmeans 11 by the processing-feed means 37. After the chuck table 36 ispositioned right below the image pick-up means 11, the semiconductorwafer 30 on the chuck table 36 becomes a state where it is located atthe coordinate position shown in FIG. 9. In this state, alignment workis carried out to check whether the dividing lines 301 formed in alattice pattern on the semiconductor wafer 30 held on the chuck table 36are parallel to the X direction and the Y direction. That is, an imageof the semiconductor wafer 30 held on the chuck table 36 is picked up bythe image pick-up means 11 to carry out image processing such as patternmatching, etc. for the alignment work. Although the dividing line 301formed front surface 30 a of the semiconductor wafer 30 faces down atthis point, as the image pick-up means 11 is constituted by the infraredilluminating means, an optical system for capturing infrared radiationand an image pick-up device (infrared CCD) for outputting an electricsignal corresponding to the infrared radiation as described above, animage of the dividing lines 301 can be picked up through the rearsurface 30 b of the semiconductor wafer 30.

Thereafter, the chuck table 36 is moved to bring a device 302 at themost left end in FIG. 9 in the top row E1 out of the devices 302 formedon the semiconductor wafer 30 to a position right below the imagepick-up means 11. Further, the upper left electrode 303 a in FIG. 9 outof the electrodes 303 (303 a to 303 j) formed on the above device 302 isbrought to a position right below the image pick-up means 11. After theimage pick-up means 11 detects the electrode 303 a in this state, itscoordinate values (a1) as first feed start position coordinate valuesare supplied to the control means 10. The control means 10 stores thecoordinate values (a1) in the random access memory (RAM) 103 as firstprocessing-feed start position coordinate values (processing-feed startposition detecting step). Since there is a predetermined space betweenthe image pick-up means 11 and the condenser 8 of the laser beamapplication means 52 in the X direction at this point, a value obtainedby adding the interval between the above image pick-up means 11 and thecondenser 8 is stored as an X coordinate value.

After the first processing-feed start position coordinate values (a1) ofthe device 302 in the top-most row E1 in FIG. 9 are detected asdescribed above, the chuck table 36 is moved a distance corresponding tothe interval between dividing lines 301 in the Y direction and moved inthe X direction to bring a device 302 at the most left end in the secondrow E2 from the top-most in FIG. 9 to a position right below the imagepick-up means 11. Further, the upper left electrode 303 a in FIG. 9 outof the electrodes 303 (303 a to 303 j) formed on the above device 302 isbrought to a position right below the image pick-up means 11. After theimage pick-up means 11 detects the electrode 303 a in this state, itscoordinate values (a2) are supplied to the control means 10 as secondprocessing-feed start position coordinate values. The control means 10stores the coordinate values (a2) in the random access memory (RAM) 103as second processing-feed start position coordinate values. Since thereis the predetermined space between the image pick-up means 11 and thecondenser 8 of the laser beam application means 52 in the X direction asdescribed above, a value obtained by adding the interval between theimage pick-up means 11 and the condenser 8 is stored as an X coordinatevalue. Thereafter, the control means 10 carries out the aboveindexing-feed and processing-feed start position detecting stepsrepeatedly up to the bottom row En in FIG. 9 to detect theprocessing-feed start position coordinate values (a3 to an) of thedevices 302 formed in the rows and store them in the random accessmemory (RAM) 103.

Next comes the laser-processing step of forming a laser-processed viahole in portions corresponding to the electrodes 303 (303 a to 303 j)formed on each device 302 of the semiconductor wafer 30. In the laserprocessing step, the processing-feed means 37 is first activated to movethe chuck table 36 so as to bring the point of the first processing-feedstart position coordinate values (a1) stored in the above random accessmemory (RAM) 103 to a position right below the condenser 8 of the laserbeam application means 52. FIG. 10 shows a state where the point of thefirst processing-feed start position coordinate values (a1) has beenpositioned right below the condenser 8. From the state shown in FIG. 10,the control means 10 controls the laser beam application means 52 toapply the processing laser beam LB from the condenser 8.

The processing conditions in the above laser processing step are set asfollows, for example.

-   -   Light source: LD excited Q switch Nd:YVO4 pulse laser    -   Wavelength: 355 nm    -   Energy density: 30 J/cm²    -   Focal spot diameter: 70 μm

When the laser processing step is carried out under the above processingconditions, a via hole having a depth of about 2 μm per one pulse of thepulse laser beam can be formed in the silicon wafer. Therefore, alaser-processed via hole 310 reaching the electrode 303 can be formed byirradiating 50 pulses of the pulse laser beam to a silicon wafer havinga thickness of 100 μm as shown in FIG. 11.

After the laser processing step is carried out at the point of the firstprocessing-feed start position coordinate values (a1) as describedabove, the processing-feed means 37 is activated to move the chuck table36 a distance corresponding to the above interval A so as to bring aportion corresponding to the electrode 303 b to a position right belowthe condenser 8 of the laser beam application means 52. Then, the abovelaser processing step is carried out. By bringing portions correspondingto all the electrodes 303 formed on the semiconductor wafer 30 to aposition right below the condenser 8 of the laser beam application means52 and carrying out the above laser processing step as described above,laser-processed via holes 310 reaching the electrodes 303 can be formedfrom the rear surface 30 b in the semiconductor wafer 30.

Although laser-processed via holes 310 reaching the electrodes 303 canbe formed from the rear surface 30 b in the semiconductor wafer 30 bycarrying out the above laser processing step, some laser-processed viaholes 310 may not reach the electrodes 303. Therefore, it is necessaryto check if the laser-processed via holes 310 have reached, theelectrodes 303 or not. In the illustrated laser beam processing machine1, the above via hole depth detector 9 is activated to carry out thestep of detecting the depth of each laser-processed via hole 310 formedin the semiconductor wafer 30.

The depth detection step is carried out by bringing the top surface(rear surface 30 b) of the semiconductor wafer 30 and eachlaser-processed via hole 310 to a position right below the condenser 8sequentially, oscillating the first detection laser beam LB1 from thefirst detection laser beam oscillation means 90 a of the first surfaceposition detection means 9 a constituting the via hole depth detector 9and the second detection laser beam LB2 from the second detection laserbeam oscillation means 90 b of the second surface position detectionmeans 9 b constituting the via hole depth detector 9, and applying themto the semiconductor wafer 30 from the condenser 8 as shown in FIGS. 12(a) and 12(b). The positioning of each laser-processed via hole 310formed in the semiconductor wafer 30 can be carried out by moving thechuck table 36 based on the coordinate data of the electrodes 303 storedin the random access memory (RAM) 103 of the control means 10 in thesame manner as in the above laser processing step.

Examples of the first detection laser beam LB1 and the second detectionlaser beam LB2 irradiated in the depth detection step are describedbelow.

-   (1) First Detection Laser Beam LB1    -   Light source: He—Ne continuous wave laser    -   Wavelength: 633 nm    -   Average output: 2 to 3 mW-   (2) Second Detection Laser Beam LB2    -   Light Source: YAG Continuous Wave Laser    -   Wavelength: 532 nm    -   Average output: 2 to 3 mW

The distance between the focal point P1 of the first detection laserbeam LB1 and the focal point P2 of the second detection laser beam LB2is adjusted by controlling the interval between the two convex lenses991 and 992 constituting the beam expander 99 as the above focal pointpositioning means. For instance, to form a via hole having a depth of100 μm and reaching the electrode 303 in the semiconductor wafer 30having a thickness of 100 μm, the distance between the focal point P1 ofthe first detection laser beam LB1 and the focal point P2 of the seconddetection laser beam LB2 is set to 90 to 100 μm. The focal point P2 ofthe second detection laser beam LB2 is set to the undersurface (frontsurface 30 a) of the semiconductor wafer 30, that is, the rear surfaceof the electrode 303 by the focal point positioning means 53 as theZ-axis moving means as shown in FIG. 12( a). Therefore, the focal pointP1 of the first detection laser beam LB1 is positioned h1 above theundersurface (front surface 30 a) of the semiconductor wafer 30.

The first detection laser beam LB1 is reflected at an area S1 and thesecond detection laser beam LB2 is reflected at an area S2 on the topsurface (rear surface 30 b) of the semiconductor wafer 30 respectively,and the reflected light of the first detection laser beam LB1 isreceived by the first light receiving device 96 a and the second lightreceiving device 97 a of the first surface position detection means 9 aas described above. Therefore, the height hx from the focal point P1 tothe top surface (rear surface 30 b) of the semiconductor wafer 30 can bedetected from the ratio (V1/V2) of the voltage value (V1) output fromthe first light receiving device 96 a to the voltage value (V2) outputfrom the second light receiving device 97 a based on the control mapshown in FIG. 4 as described above. The thickness H1 of thesemiconductor wafer 30 can be obtained by adding this height hx to theheight h1 from the undersurface (front surface 30 a) of thesemiconductor wafer 30 to the focal point P1 (H1=h1+hx). Although thereflected light of the second detection laser beam LB2 is received bythe first light receiving device 96 b and the second light receivingdevice 97 b of the second surface position detection means 9 b asdescribed above, as the area of the reflected light is very large, theratio (V1/V2) of the voltage value (V1) output from the first lightreceiving device 96 b to the voltage value (v2) output from the secondlight receiving device 97 b is extremely high, whereby the control means10 makes it invalid as abnormal data. Thus, the thickness H1 of thesemiconductor wafer 30 detected by the first surface position detectionmeans 9 a based on the first detection laser beam LB1 is stored in therandom access memory (RAM) 103 of the control means 10.

Next, when the laser-processed via holes 310 formed in the semiconductorwafer 30 are brought to a position right below the condenser 11sequentially and the first detection laser beam LB1 and the seconddetection laser beam LB2 are applied to the top surface (rear surface 30b) of the semiconductor wafer 30, the focal point P1 of the firstdetection laser beam LB1 is located h1 (at a lower position of a depthhx from the top surface (rear surface 30 b) of the semiconductor wafer30) above the undersurface (front surface 30 a) of the semiconductorwafer 30 as shown in FIG. 12( a). Meanwhile, the focal point P2 of thesecond detection laser beam LB2 is located at the undersurface (frontsurface 30 a) of the semiconductor wafer 30. The first detection laserbeam LB1 is reflected at an area S1 and the second detection laser beamLB2 is reflected at an area S2 on the laser-processed via hole 310formed in the semiconductor wafer 30 respectively on the bottom surfaceof the laser-processed via hole 310, and the reflected light of thesecond detection laser beam LB2 is received by the first light receivingdevice 96 b and the second light receiving device 97 b of the secondsurface position detection means 9 b as described above. Therefore, theheight T from the focal point P2 to the bottom surface of thelaser-processed via hole 310 formed in the semiconductor wafer 30 can bedetected from the ratio (V1/V2) of the voltage value (V1) output fromthe first light receiving device 96 b to the voltage value (V2) outputfrom the second light receiving device 97 b based on the control mapshown in FIG. 4. This height T is from the undersurface (front surface30 a) of the semiconductor wafer 30 to the bottom surface of thelaser-processed via hole 310 because the focal point P2 is located atthe undersurface (front surface 30 a) of the semiconductor wafer 30.Since the laser-processed via hole 310 reaches the undersurface (frontsurface 30 a) of the semiconductor wafer 30, that is, the electrode 303in the illustrated embodiment shown in FIG. 12( b), the height T becomesnil (0). Meanwhile, although the reflected light of the first detectionlaser beam LB1 is received by the first light receiving device 96 a andthe second light receiving device 97 a of the first surface positiondetection means 9 a, as the focal spot area of the reflected light isvery large, the ratio (V1/V2) of the voltage value (V1) output from thefirst light receiving device 96 a to the voltage value (V2) output fromthe second light receiving device 97 a is extremely high, whereby thecontrol means 10 makes it invalid as abnormal data. The height T (heightfrom the undersurface (front surface 30) of the semiconductor wafer 30to the bottom surface of the laser-processed via hole 310) from thefocal point P2 to the bottom surface of the laser-processed via hole 310detected by the second surface position detection means 9 b based on thesecond detection laser beam LB2 is stored in the random access memory(RAM) 103 of the control means 10.

The control means 10 obtains the depth H2 of the laser-processed viahole 310 formed in the semiconductor wafer 30 based on the detectedthickness H1 of the semiconductor wafer 30 and the height T (height fromthe undersurface (front surface 30 a) of the semiconductor wafer 30 tothe bottom surface of the laser-processed via hole 310) from the focalpoint P2 to the bottom surface of the laser-processed via hole 310. Thatis, the control means 10 obtains the depth H2 of the laser-processed viahole 310 formed in the semiconductor wafer 30 by subtracting the heightT (height from the undersurface (front surface 30 a) of thesemiconductor wafer 30 to the bottom surface of the laser-processed viahole 310) from the focal point P2 to the bottom surface of thelaser-processed via hole 310 from the thickness H1 of the semiconductorwafer 30 (H2=H1−T). When the depth H2 of the laser-processed via hole310 is equal to the thickness H1 of the semiconductor wafer 30, thelaser-processed via hole 310 reaches the bottom surface, that is, therear surface of the electrode 303. Meanwhile, when the value of thedepth H2 of the laser-processed via hole 310 is positive, thelaser-processed via hole 310 does not reach the bottom surface. Theheight T (unprocessed thickness) from the focal point P2 to the bottomsurface of the above laser-processed via hole 310 and the depth H2 ofthe laser-processed via hole 310 are stored in the random access memory(RAM) 103 of the control means 10.

The above depth detection step is carried out on all the laser-processedvia holes 310 formed in the semiconductor wafer 30 and then, the controlmeans 10 obtains the heights T (unprocessed thicknesses) from the focalpoint P2 to the bottom surfaces of the laser-processed via holes 310 andthe depths H2 of the laser-processed via holes 310 based on the controlmap shown in FIG. 4 to prepare the decision map of the laser-processedvia holes 310 as shown in FIG. 13.

The control means 10 stores data on the reflectance of silicon formingthe wafer and the reflectance of a metal such as copper forming theelectrodes in the predetermined area of the random access memory (RAM)103 and judges whether the output value from the first light receivingdevice 96 b of the second surface position detection means 9 b is avalue corresponding to the reflectance of the electrodes 303. That is,as the reflectance of silicon is 57.48% and the reflectance of copperforming the electrodes 303 is 43.27%, the quantity of light received bythe first light receiving device 96 b of the second surface positiondetection means 9 b changes according to whether the laser-processed viahole 310 reaches the electrode 303 or not. Therefore, when the outputvalue from the first light receiving device 96 b is a valuecorresponding to the reflectance of copper, it is known that thelaser-processed via hole 310 formed in the semiconductor wafer 30reaches the electrode 303 surely. Consequently, even if the depth H2 ofthe laser-processed via hole 310 is calculated as not equal to thethickness H1 of the semiconductor wafer 30, it can be judged that thelaser-processed via hole 310 reaches the electrode 303.

After the decision map shown in FIG. 13 is prepared as described above,the control means 10 sets the number of re-processing pulses forlaser-processed via holes 310 which have been judged as not reaching theelectrodes 303 (303 f on E1 and 303 b on En in the embodiment shown inFIG. 13). That is, since a laser-processed via hole having a depth ofabout 2 μm can be formed with one pulse of the pulse laser beam underthe above processing conditions, the number of re-processing pulses isset to 5 for the laser-processed via hole 310 which should reach theelectrode 303 f on E1 because it must be made 10 μm deeper and thenumber of re-processing pulses is set to 10 for the laser-processed viahole 310 which should reach the electrode 303 b on En because it must bemade 20 μm deeper. The decision map prepared as described above isdisplayed on the display means 100.

After the numbers of re-processing pulses are set in the decision mapshown in FIG. 13, the control means 10 moves the chuck table 36 to bringthe point of the coordinate values of the electrode 303 f on E1 to aposition right below the condenser 8 and applies 5 pulses of theprocessing pulse laser beam LB and then, moves the chuck table 36 tobring the point of the coordinate values of the electrode 303 b on Enright below the condenser 8 and applies 10 pulses of the processingpulse laser beam LB. As a result, the laser-processed via hole 310 whichhas been determined not to reach the electrode 303 f on E1 and thelaser-processed via hole 310 which has been determined not to reach theelectrode 303 b on En are re-processed so as to reach these electrodes303.

Another embodiment of the via hole depth detector 9 according to thepresent invention will be described with reference to FIG. 14. In thevia hole depth detector 9 shown in FIG. 14, a common detection laserbeam oscillation means for oscillating a detection laser beam isprovided, parts substantially the same as the constituent parts of thevia hole depth detector 9 shown in FIG. 2 are given the same referencenumbers, and their detailed descriptions are omitted.

The via hole depth detector 9 shown in FIG. 14 comprises the firstsurface position detection means 9 a and the second surface positiondetection means 9 b, and there is further provided a common detectionlaser beam oscillation means 900. As this detection laser beamoscillation means 900 may be used a He—Ne laser oscillator foroscillating a detection laser beam LB3 having a wavelength of 633 nm,like the first detection laser beam oscillation means 90 a of the viahole depth detector 9 shown in FIG. 2. A deflection plate 901 and adeflection beam splitter 902 are interposed between the detection laserbeam oscillation means 900 and the first non-deflection beam splitter 92a of the first surface position detection means 9 a and the secondnon-deflection beam splitter 92 b of the second surface positiondetection means 9 b. The P wave LBP and S wave LBS of the detectionlaser beam LB3 oscillated from the detection laser beam oscillationmeans 900 are limited to predetermined angles by the deflection plate901. The P wave LBP limited to a predetermined angle by the deflectionplate 901 is directed toward the first non-deflection beam splitter 92 ainstalled in the first optical path A of the first surface positiondetection means 9 a by the deflection beam splitter 902. Meanwhile, theS wave LBS obtained by the deflection plate 901 is directed toward thesecond non-deflection beam splitter 92 b installed in the second opticalpath B of the second surface position detection means 9 b by thedeflection beam splitter 902.

The P wave LBP of the detection laser beam LB3 directed toward the firstnon-deflection beam splitter 92 a of the first surface positiondetection means 9 a is converged by the condenser lens 81 through thefirst non-deflection beam splitter 92 a, the dichroic half mirror 91 a,the synthetic beam splitter 91 and the direction changing mirror 7 as inthe via hole depth detector 9 shown in FIG. 2. The P wave LBP of thedetection laser beam LB3 converged as described above is reflected onthe front surface (top surface) of the workpiece held on the chuck table36 shown in FIG. 1, and its reflected light reaches the pass filter 903a through the condenser lens 81, the direction changing mirror 7, thesynthetic beam splitter 91, the dichroic half mirror 91 a and the firstnon-deflection beam splitter 92 a as shown by a broken line in FIG. 14.The reflected light of the S wave LBS of the detection laser beam LB3also reaches the pass filter 903 a through the same route as the P waveLBP of the detection laser beam LB3 as will be described later. Sincethis pass filter 903 a is so constituted as to transmit only the P wave,the reflected light of the S wave LBS is cut off by the pass filter 903a. Therefore, only the reflected light of the P wave LBP of thedetection laser beam LB3 passes through the pass filter 903 a andreaches the non-deflection auxiliary beam splitter 94 a.

The reflected light of the P wave LBP of the detection laser beam LB3which has reached the non-deflection auxiliary beam splitter 94 a issplit into the fifth optical path E and the six optical path F, like inthe first surface position detection means 9 a shown in FIG. 2. Thereflected light split into the fifth optical path E is 100% converged bythe condenser lens 95 a and received by the first light receiving device96 a. And, the first light receiving device 96 a supplies a voltagesignal corresponding to the quantity of its received light to thecontrol means 10. Meanwhile, the reflected light of the P wave LBP ofthe detection laser beam LB3 split into the sixth optical path F isconverged unidimensionally by the cylindrical lens 981 a, restricted tothe predetermined unit length by the unidimensional mask 982 a of thelight receiving area restricting means 98 a and received by the secondlight receiving device 97 a. And, the second light receiving device 97 asupplies a voltage signal corresponding to the quantity of its receivedlight to the control means 10.

Thus, the control means 10 can obtain the thickness H1 of thesemiconductor wafer 30 based on the voltage signals supplied from thefirst light receiving device 96 a and the second light receiving device97 a of the first surface position detection means 9 a as describedabove.

The S wave LBS of the detection laser beam LB3 deflected toward thesecond non-deflection beam splitter 92 b of the second surface positiondetection means 9 b is converged by the condenser lens 81 through thesecond non-deflection beam splitter 92 b, the beam expander 99, thesynthetic beam splitter 91 and the direction changing mirror 7, like inthe via hole depth detector 9 shown in FIG. 2. The S wave LBS of thedetection laser beam LB3 converged as described above is reflected onthe front surface (top surface) of the workpiece held on the chuck table36 shown in FIG. 1, and its reflected light reaches the pass filter 903b through the condenser lens 81, the direction changing mirror 7, thesynthetic beam splitter 91, the beam expander 99, the secondnon-deflection beam splitter 92 b and the direction changing mirror 904as shown by a broken line in FIG. 14. The reflected light of the P waveLBP of the detection laser beam LB3 also reaches the pass filter 903 bthrough the same route as the S wave LBS of the detection laser beamLB3. Since this pass filter 903 b is so constituted as to transmit onlythe S wave LBS, the reflected light of the P wave LBP is cut off by thepass filter 903 b. Therefore, only the reflected light of the S wave LBSof the detection laser beam LB3 passes through the pass filter 903 b andreaches the non-deflection auxiliary beam splitter 94 b.

The reflected light of the S wave LBS of the detection laser beam LB3which has reached the non-deflection auxiliary beam splitter 94 b issplit into the eighth optical path J and the ninth optical path K. Thereflected light split into the eighth optical path J is 100% convergedby the condenser lens 95 b and received by the first light receivingdevice 96 b. And, the first light receiving device 96 b supplies avoltage signal corresponding to the quantity of its received light tothe control means 10. Meanwhile, the reflected light of the S wave LBSof the detection laser beam LB3 split into the ninth optical path K isconverged unidimensionally by the cylindrical lens 981 b, restricted tothe predetermined unit length by the unidimensional mask 982 b of thelight receiving area restricting means 98 b and received by the secondlight receiving device 97 b. And, the second light receiving device 97 bsupplies a voltage signal corresponding to the quantity of its receivedlight to the control means 10.

Thus, the control means 10 can obtain the thickness H1 of thesemiconductor wafer 30 and the height T from the undersurface (frontsurface 30 a) of the semiconductor wafer 30 to the bottom surface of thelaser-processed via hole 310 based on the voltage signals supplied fromthe first light receiving device 96 a and the second light receivingdevice 97 a of the first surface position detection means 9 a and thefirst light receiving device 96 b and the second light receiving device97 b of the second surface position detection means 9 b, as describedabove. Thereafter, the control means 10 obtains the depth H2 of thelaser-processed via hole 310 by subtracting the height T from theundersurface (front surface 30 a) of the semiconductor wafer 30 to thebottom surface of the laser-processed via hole 310 from the thickness H1of the semiconductor wafer 30 (H2=H1−T) and prepares the decision map ofthe laser-processed via holes 310 as shown in FIG. 13 based on theheights T from the undersurface (front surface 30 a) of thesemiconductor wafer 30 to the bottom surfaces of the laser-processed viaholes 310 and the depths H2 of the laser-processed via holes 310.

1. A via hole depth detector for detecting the depth of a via holeformed in a workpiece held on the holding surface of a chuck table,comprising: a first surface position detection means which comprises afirst detection laser beam oscillation means for oscillating a firstdetection laser beam having a predetermined wavelength to a firstoptical path and detects the height position of an illuminated portionof the workpiece based on the reflected light of the first detectionlaser beam; a second surface position detection means which has a seconddetection laser beam oscillation means for oscillating a seconddetection laser beam having a wavelength different from the wavelengthof the first detection laser beam to a second optical path and detectsthe height position of an illuminated portion of the workpiece based onthe reflected light of the second detection laser beam; a synthetic beamsplitter for guiding the first detection laser beam oscillated to thefirst optical path and the second detection laser beam oscillated to thesecond optical path to a third optical path; a condenser which isinstalled in the third optical path and converges the first detectionlaser beam and the second detection laser beam to apply them to theworkpiece held on the chuck table; a focal point positioning means whichis installed in the first or second optical path and changes theposition of the focal point of the first detection laser beam or thesecond detection laser beam; and a control means for obtaining the depthof a via hole formed in the workpiece based on a detection valueobtained by the first surface position detection means and a detectionvalue obtained by the second surface position detection means.
 2. Thevia hole depth detector according to claim 1, wherein the first surfaceposition detection means comprises a first non-deflection beam splitterwhich is installed in the first optical path and guides light reflectedfrom the workpiece to a fourth optical path, a pass filter which isinstalled in the fourth optical path and transmits only reflected lighthaving a wavelength corresponding to that of the first detection laserbeam out of the reflected light split by the first non-deflection beamsplitter, a non-deflection auxiliary beam splitter for splitting thereflected light passing through the pass filter into a fifth opticalpath and a sixth optical path, a first light receiving device whichreceives 100% of the reflected light split into the fifth optical pathby the non-deflection auxiliary beam splitter, a second light receivingdevice which receives the reflected light split into the sixth opticalpath by the non-deflection auxiliary beam splitter, and a lightreceiving area restricting means which is installed in the sixth opticalpath and restricts the light receiving area of the reflected light to bereceived by the second light receiving device; the second surfaceposition detection means comprises a second non-deflection beam splitterwhich is installed in the second optical path and guides light reflectedfrom the workpiece to a seventh optical path, a pass filter which isinstalled in the seventh optical path and transmits only reflected lighthaving a wavelength corresponding to that of the second detection laserbeam out of the reflected light guided by the second non-deflection beamsplitter, a non-deflection auxiliary beam splitter for splitting thereflected light passing through the pass filter into an eighth opticalpath and a ninth optical path, a first light receiving device whichreceives 100% of the reflected light split into the eighth optical pathby the non-deflection auxiliary beam splitter, a second light receivingdevice which receives the reflected light split into the ninth opticalpath by the non-deflection auxiliary beam splitter, and a lightreceiving area restricting means which is installed in the ninth opticalpath and restricts the light receiving area of the reflected light to bereceived by the second light receiving device; and the control meanscalculates the ratio of the quantity of light received by the firstlight receiving device to the quantity of light received by the secondlight receiving device of the first surface position detection meansbased on detection signals from the first light receiving device and thesecond light receiving device to obtain the height position of the topsurface of the workpiece or the height position of the bottom surface ofa via hole based on the ratio, calculates the ratio of the quantity oflight received by the first light receiving device to the quantity oflight received by the second light receiving device of the secondsurface position detection means based on detection signals from thefirst light receiving device and the second light receiving device toobtain the height position of the bottom surface of the via hole or theheight position of the top surface of the workpiece based on the ratio,and obtains the depth of the via hole formed in the workpiece based onthe height position of the top surface of the workpiece and the heightposition of the bottom surface of the via hole.
 3. The via hole depthdetector according to claim 2, wherein the control means comprises amemory having a storage area for storing a control map showing therelationship between the ratio of the quantity of light received by thefirst light receiving device to the quantity of light received by thesecond light receiving device of the first surface position detectionmeans or the second surface position detection means and the heightposition of a portion illuminated by the first detection laser beam orthe second detection laser beam, calculates the ratio of the quantity oflight received by the first light receiving device to the quantity oflight received by the second light receiving device of the first surfaceposition detection means based on detection signals from the first lightreceiving device and the second light receiving device to obtain theheight position of the top surface of the workpiece or the heightposition of the bottom surface of the via hole by collating the ratiowith the control map, calculates the ratio of the quantity of lightreceived by the first light receiving device to the quantity of lightreceived by the second light receiving device of the second surfaceposition detection means based on detection signals from the first lightreceiving device and the second light receiving device to obtain theheight position of the bottom surface of the via hole or the heightposition of the top surface of the workpiece by collating the ratio withthe control map, and obtains the depth of the via hole formed in theworkpiece based on the height position of the top surface of theworkpiece and the height position of the bottom surface of the via hole.4. The via hole depth detector according to claim 2, wherein the memoryof the control means has a storage area for storing data on thereflectances of a plurality of substances and the control means judgeswhether the via hole formed in the workpiece reaches some othersubstance from the processed substance based on the amount of lightreceived by the first light receiving device of the first surfaceposition detection means or the second surface position detection means.5. A via hole depth detector for detecting the depth of a via holeformed in a workpiece held on the holding surface of a chuck table,comprising: a detection laser beam oscillation means for oscillating adetection laser beam; a deflection beam splitter for splitting thedetection laser beam oscillated by the detection laser beam oscillatorinto a P wave and an S wave and guiding them to a first optical path anda second optical path, respectively; a synthetic beam splitter forguiding the P wave and S wave of the detection laser beam split by thedeflection beam splitter to a third optical path; a condenser which isinstalled in the third optical path and converges the P wave and S waveof the detection laser beam to apply them to the workpiece held on thechuck table; a focal point positioning means which is installed in thefirst or second optical path and changes the focal point position of theP wave or S wave of the detection laser beam; a first surface positiondetection means for detecting the height position of an illuminatedportion of the workpiece based on the reflected light of the P wave or Swave of the detection laser beam applied to the workpiece from thecondenser; second surface position detection means for detecting theheight position of an illuminated portion of the workpiece based on thereflected light of the S wave or P wave of the detection laser beamapplied to the workpiece from the condenser; and a control means forobtaining the depth of a via hole formed in the workpiece based on adetection value obtained by the first surface position detection meansand a detection value obtained by the second surface position detectionmeans.
 6. The via hole depth detector according to claim 5, wherein thefirst surface position detection means comprises a first non-deflectionbeam splitter which is installed in the first optical path and guideslight reflected from the workpiece to a fourth optical path, a passfilter which is installed in the fourth optical path and transmits onlythe P wave or S wave of the detection laser beam out of the reflectedlight guided by the first non-deflection beam splitter, a non-deflectionauxiliary beam splitter for splitting the reflected light passingthrough the pass filter into a fifth optical path and a sixth opticalpath, a first light receiving device which receives 100% of thereflected light split into the fifth optical path by the non-deflectionauxiliary beam splitter, a second light receiving device which receivesthe reflected light split into the sixth optical path by thenon-deflection auxiliary beam splitter, and a light receiving arearestricting means which is installed in the sixth optical path andrestricts the light receiving area of the reflected light to be receivedby the second light receiving device; the second surface positiondetection means comprises a second non-deflection beam splitter which isinstalled in the second optical path and guides light reflected from theworkpiece to a seventh optical path, a pass filter which is installed inthe seventh optical path and transmits only the S wave or P wave of thedetection laser beam out of the reflected light guided by the secondnon-deflection beam splitter, a non-deflection auxiliary beam splitterfor splitting the reflected light passing through the pass filter intoan eighth optical path and a ninth optical path, a first light receivingdevice which receives 100% of the reflected light split into the eighthoptical path by the non-deflection auxiliary beam splitter, a secondlight receiving device which receives the reflected light split into theninth optical path by the non-deflection auxiliary beam splitter, and alight receiving area restricting means which is installed in the ninthoptical path and restricts the light receiving area of the reflectedlight to be received by the second light receiving device; and thecontrol means calculates the ratio of the quantity of light received bythe first light receiving device to the quantity of light received bythe second light receiving device of the first surface positiondetection means based on detection signals from the first lightreceiving device and the second light receiving device to obtain theheight position of the top surface of the workpiece or the heightposition of the bottom surface of a via hole based on the ratio,calculates the ratio of the quantity of light received by the firstlight receiving device to the quantity of light received by the secondlight receiving device of the second surface position detection meansbased on detection signals from the first light receiving device and thesecond light receiving device to obtain the height position of thebottom surface of the via hole or the height position of the top surfaceof the workpiece based on the ratio, and obtains the depth of the viahole formed in the workpiece based on the height position of the topsurface of the workpiece and the height position of the bottom surfaceof the via hole.
 7. The via hole depth detector according to claim 6,wherein the control means comprises a memory having a storage area forstoring a control map indicating the relationship between the ratio ofthe quantity of light received by the first light receiving device tothe quantity of light received by the second light receiving device ofthe first surface position detection means or the second surfaceposition detection means and the height position of a portionilluminated by the P wave or S wave of the detection laser beam,calculates the ratio of the quantity of light received by the firstlight receiving device to the quantity of light received by the secondlight receiving device of the first surface position detection meansbased on detection signals from the first light receiving device and thesecond light receiving device to obtain the height position of the topsurface of the workpiece or the height position of the bottom surface ofthe via hole by collating the ratio with the control map, calculates theratio of the quantity of light received by the first light receivingdevice to the quantity of light received by the second light receivingdevice of the second surface position detection means based on detectionsignals from the first light receiving device and the second lightreceiving device to obtain the height position of the bottom surface ofthe via hole or the height position of the top surface of the workpieceby collating the ratio with the control map, and obtains the depth ofthe via hole formed in the workpiece based on the height position of thetop surface of the workpiece and the height position of the bottomsurface of the via hole.
 8. The via hole depth detector according toclaim 6, wherein the memory of the control means has a storage area forstoring data on the reflectances of a plurality of substances, and thecontrol means judges whether the via hole formed in the workpiecereaches another substance from the processed substance based on thequantity of light received by the first light receiving device of thefirst surface position detection means or the second surface positiondetection means.